1. Field of the Invention
The present invention relates to a photochromic material and an optical storage medium using the same.
2. Description of the Prior Art
Substances which display reversible color changes upon exposure to light are collectively known as photochromic materials. Spiropyran compounds constitute one of the most intensively studied types of photochromic material.
Many spiropyran compounds have already been reported in the literature. For example, the colorless spiropyran compound (A) of the following formula is transformed into the red compound merocyanine (B) by irradiation with ultraviolet rays of wavelength approximately 340 nm. The compound (B) reverts to the form (A) if irradiated with visible light with a wavelength of approximately 580 nm. ##STR2##
Optical storage media can be prepared by utilizing these photochromic materials which change their structures by irradiation. In order to miniaturize devices containing optical storage media, the use of semiconductor laser as a light source is generally desirable. The semiconductor lasers employed in optical storage devices ordinarily emit light in a wavelength region in the neighborhood of 700 nm, and therefore the colored form of photochromic materials used for such purposes should desirably possess absorption sensitivity in this wavelength region.
In general, either colorless forms or colored forms of photochromic materials are thermally unstable. For example, in the case of the spiropyran compound described above, colored form B is less stable than colorless form A, and the colored form reverts to the colorless form even though it is allowed to stand in the dark at room temperature. In order to overcome this problem, for example, an alkyl group is introduced into the spiropyran skeleton in Japanese Laid-Open Patent Publication No. 61-116353. When an LB film used as an optical medium is formed from the spiropyran compound into which an alkyl group is introduced, a molecular aggregate is formed. Therefore, the stability increases when the compound becomes a colored form. The colored form of the spiropyran compound, in which a molecular aggregate is formed, is highly stable compared with the conventional one, but it does not possess high sensitivity with respect to an oscillation range of a semiconductor laser (i.e., 650 nm or more). Accordingly, it is necessary to use a large amount of spiropyran compound in order to produce a device having a predetermined performance, making it difficult to miniaturize the device.